1. Field of the Invention
The present invention relates to a spread spectrum method, and more particularly to a rake receiver for synthesizing in a time rage, maximum ratios of signals with various time delays, which are received at an antenna, due to the multiple-reflection of a propagation path, as a reception diversity method in a multi-path environment.
2. Description of the Related Art
Spread spectrum or spread spectrum communication is used in a wide area as the most fundamental technology of mobile communication.
In a Direct Sequence (DS) method used as the simplest model of spread spectrum communication, a transmission side modulates as a spread signal, a PN signal with a chip width Tc that is about 1/10 to 1/100 of a cycle T of an information signal to be transmitted, in other words, the PN signal is multiplied by the information signal and the spectrum is expanded, so that the signal is finally transmitted to a reception side.
On the reception side, a signal component is detected by a de-spreading process from among signals that are buried in a noise•Fundamentally, a de-spreading process is to multiply the same PN signal having the same phase as that of the PN signal in a reception signal by the reception signal, thereby demodulating the multiplied PN signal.
In a multi-path environment where many reflection waves exist in addition to direct waves, however, a correct signal component needs to be detected by appropriately synthesizing the reception signals with various time delays.
As one of such conventional technologies, there is a rake method. A rake method is a diversity method for gathering pieces of signal power, that are dispersed like a “rake” by the delay dispersion of a propagation path, thereby synthesizing maximum ratios.
In a conventional rake receiver, a plurality of path timings at which a multi-path arrives are detected using a known signal, and these timings are informed to a demodulator. The demodulator executes a de-spreading process at these timings, and demodulates a desired signal by synthesizing the signals of a multi-path.
FIG. 1 is a block diagram showing a whole constitution of the conventional example of a rake receiver, for example, as a mobile communication terminal. In FIG. 1, the receiver is configured by an antenna 100; a radio reception unit 101; an A/D conversion unit 102; a searcher 103 for detecting timings of a plurality of paths of a multi-path; a de-spreading timing generation unit and a de-spreading unit 104 for executing a de-spreading process for a plurality of paths corresponding to the timings of a plurality of paths that are detected by the searcher 103; a signal synthesis unit 105 for synthesizing signals of a plurality of paths; a signal process unit 106 such as a channel codec, etc. for receiving a signal from the signal synthesis unit 105, and for outputting the reception signal to a display, speaker, etc.; a level measurement unit 107 for measuring levels etc. of the reception signals of a plurality of paths, for forwarding reliability information and signal level information to the signal synthesis unit 105, and also for forwarding transmission electric power control information to be transmitted to a base station, to a transmission unit; and a transmission unit 108 for transmitting the input from a keyboard or a microphone corresponding to the control information from the level measurement unit 107.
FIG. 2 is a block diagram showing the detailed constitution of a signal demodulation unit of FIG. 1, in other words, a de-spreading timing generation unit and a de-spreading unit 104 of FIG. 1. In FIG. 2, the demodulation unit is composed of a spreading code generation unit 110, a plurality of delay control units 111, and a plurality of correlators 112 which correspond to the delay control units.
The spreading code generation unit 110 generates codes to be used for a de-spreading process. The delay control unit 111 controls the delay actions of a plurality of correlators 112 corresponding to the respective timings t1 to tN of a multi-path, which are detected by the searcher 103. Each correlator 112 de-spreads the reception signals output from the A/D conversion unit 102 corresponding to the de-spreading timing that is controlled by the delay control unit 111, and it assigns de-spread signals 1 to N to the signal synthesis unit 105. Then, the signal synthesis unit 105 outputs a demodulation signal that is obtained by synthesizing these de-spread signals. In each de-spread signal, a signal used for channel estimation corresponding to the propagation path coefficient of each multi-path is included.
FIG. 3 is a block diagram showing the constitution example of a MMSE (Minimum Mean Square Error) receiver as another example of a signal demodulation unit. In this MMSE receiver, a MMSE weight coefficient generation unit 123 generates a weight coefficient used for weight-synthesizing a channel estimation value, for example, a reference signal that is obtained from a known signal transmitted from the transmission side while being included in a transmission signal, and the output of a transversal filter 124 made by the delay line with a tap. In other words, in order to minimize the mean square of the synthesized signal and the reference signal, the output of a matched filter 122 for instantaneously detecting a correlation value utilizing a CCD (Charge Coupled Code) element is weighted.
As mentioned above, for example, in FIG. 2, a de-spreading process is executed using the timing of each path of a multi-path. In the case that a de-spreading process is executed at a certain timing, all the signals corresponding to the paths other than this timing become interference. In the case that an orthogonal spreading code is used for a plurality of channels at a descent link from the base station of a CDMA (Code Division Multiple Access) method, there is a problem that the reception characteristics deteriorate due to the multi-path interference.
FIG. 4 is a diagram explaining such multi-path interference. More specifically, FIG. 4 is a diagram explaining the multi-path interference in a propagation environment where signals of two paths arrive at a movable body from the base station.
In the case that a spread signal is multiplexed to be transmitted from a base station to a movable body using an orthogonal spread code, if a de-spreading process is executed at the timing of path A, the de-spreading timing is correct for a signal which arrives via the path A. Further, since the multiplexed spread signals are orthogonal each other, desired signals remain, and no interference ideally occurs.
If a de-spreading process is executed for a signal that arrives via path B, at the timing of path A, however, since a de-spreading timing is not correct, interference occurs to all the signal components including the desired signal component. Even in the case that a de-spreading process is executed for a signal that arrives via path A at the timing of path B, interference similarly occurs to all the signals transmitted via path A. There is a problem that the multi-path interference increases in the case that the electric power of all the signals that are included in a multiplexed signal is bigger than that of a desired signal.